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NEW YORK (CNN) – The World Health Organization issued a more urgent warning Wednesday about the spread of the potentially deadly Middle East respiratory syndrome, or MERS.

18 countries, including the United States, are now reporting cases. Experts say there are some possible ways to detect the virus.

Health officials say hundreds of people may have been exposed to the MERS virus by flying on planes, within the United States, with two MERS patients who are now in Florida and Indiana.

They’re both healthcare workers who came to the U.S. after being infected in Saudi Arabia.

One woman who flew on the same flight as a sick MERS patient told CNN affiliate WKMG, she was informed by her state health agency of her potential exposure.

“They informed me that there was a confirmed case of the MERS virus from my flight from Atlanta to Orlando. I was really scared.”

The woman says neither she nor her husband have symptoms of MERS.

The two MERS patients confirmed in the U.S. are reported to be getting better, but what if someone with MERS got out into a major city?

Experts say it does take sustained, close contact with a patient, to get it, but they also say- MERS, like ‘SARS,’ is worrisome.

“With SARS, it spread so far because people carrying the disease from Asia went to many different parts of the world and unfortunately when they got sick and went into hospitals or were taken care of by family members, they were able to infect people who were at close range.”

Is this a line of defense? A thermal-imaging camera is used to try to detect elevated body temperatures, high fevers possibly associated with MERS.

It was used Wednesday at a conference attended by Defense Secretary Chuck Hagel in Saudi Arabia, where MERS originated. Similar devices are being used at airports in Asia and the Middle East.

“It’s displayed on our camera as a colorized image, and in this particular image we see cold areas that are black-blue, to medium areas that are purple to orange, and then the warmer or higher temperature areas being yellow to white.”

Can these cameras spot MERS? “They can’t diagnose MERS with a fever-scanner. The probability that someone with a fever actually has MERS vs. the numerable other things they could have, make it a very impractical tool in this current setting.”

Health officials say some people who really have MERS could go undetected by a scanner because some may not have fevers yet.

The incubation period for MERS is 2 to 14 days. So the World Health Organization says it’s not recommending that agencies use thermal imaging-cameras.